Irrigation systems are well known for applying water to turf areas for maintaining the turf or vegetation growing there. Such systems include sprinklers distributed over the area, valves for controlling the flow of pressurized water to the sprinklers from underground water supply pipes, and an irrigation controller for operating the valves in accordance with a desired irrigation cycle stored in the controller. The controller often includes solid state logic means which allows the user to input and store practically any desired irrigation cycle.
Many known irrigation controllers operate automatically as time elapses to turn the sprinkler valves on and off at designated intervals without taking actual environmental conditions, i.e. actual soil moisture, into account. However, this is wasteful and unduly expensive as it uses water which may not be needed. In some cases, the irrigation controller includes a rain switch to stop an irrigation cycle when it is raining. While useful, this is not an exact measure of soil moisture and can potentially stop an irrigation cycle that may be needed despite the rain.
Interest has grown recently in soil moisture sensing technology in which the soil moisture level is measured directly and is used to control the irrigation process. Various types of soil moisture sensors have been proposed, including some which interface directly with an irrigation controller or valve to apply a control function. Such sensors usually provide a soil moisture reading when an input voltage is applied to them.
However, one drawback of many such sensors is their tendency to rather quickly degrade or corrode. In Applicant's opinion, the electrical power applied to them induces an eletrochemical reaction which eventually causes an increase in sensor resistance, rendering the sensor virtually useless. While much effort has been directed to sensor construction and design, not much effort has gone into looking at the control circuits used in combination with the sensor for a solution to the corrosion problem.
In addition, the irrigation industry has been somewhat reluctant to widely adopt moisture sensing technology due to a fear that it might fail and harm the area being irrigated. The prevailing thought is that it is better to apply too much water than not enough. This fear has been intensified because prior art control units for use with moisture sensors are little more than "black boxes". One basically has to trust that the sensor and control unit are performing properly. The only external check most users have is visual observation of the soil condition. However, users often misinterpret such visual observation and interrupt or terminate the use of the soil moisture sensor prematurely.